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Date: Sat, 21 Feb 2009 12:02:00 -0800
From: Scott Tilley <sthed475@telus.net>
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Subject: Re: LF: Wanted - non-random PN sequence generator - so Off Topic
 it 	shouldn't be allowed here!
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Hi Andy

Why not simply OR the xx% duty cycle with the PN sequence and suppress 
repeated TX sequences in a row? This should create something code 
compact and simple that lets you sleep at night that can be pretty easy 
to make appear 'random.'  Also add the ability to your code to allow the 
user to select their own seed word.

A 256bit LFSR sequence run over a WSPR period would take along time for 
someone to figure out a pattern. Larger sequences would only take abit 
more effort to code and if your writing in C no real effort at all.

I think Joe K1JT lost some sleep over this as well.  You should drop him 
a line as he is most helpful about such dialog.

I ran into similar aesthetic issues simulating fire light with  LFSRs  
years  ago and  found that  many of them  operating in tangent with each 
other produced very layered and real appearing effects.

Another solution I have tried is an ADC or clipper/clamp circuit on an 
input of the MCU and an inductorless Chua's circuit driving it... Period 
three implies Chaos. 
http://www.cpdee.ufmg.br/~MACSIN/services/papers/toraguel.pdf

gl es 73
Scott
VE7TIL

 

Andy Talbot wrote:
> Scott:
> Its precisely that an LFSR does not give what is needed that made me
> start thinking along these lines.   For one thing, LFSRs are only
> random if considered over the whole repeat length, viz  2 ^ N - 1 bit
> for a maximal length sequence.   They will have one single run of
> exactly N ones, and N-1 zeroes, and a predicatable numbers of runs of
> fewer successive equal bits.  It is a well established annoyance with
> these that short register lengths cannot be used to generate
> half-decent pseudo randomness
>
> What I was idly speculating on was a straightforward way of generating
> a semi-random sequence with deliberately limited continuous runs of
> ones or zeroes.  By forcing this aspect, the sequence cannot possibly
> be random. A ideal sequence would probably have very little
> statistical randomness at all.  But it sems to have turned out to be
> not at all straightforward.
>
> Eg 33% duty cycle with no randomisation
>       001001001001001001001001001001
> A true PN sequence at 33% could be like
>       010000111100100000001100101000
> which has an excessively long sequence of ones for WSPR Tx purposes
> A more optimised pattern might be
>       001010100001010100100010001010
> Apart from one bit, this one is actually a smaller pattern repeated.
>
> Which all comes down to the idea that its easiest to just generate (by
> inspection !) a set of precomputed tables - but that just feels like
> cheating and I feel dissatisfied with it :-)
>
> Andy
> www.g4jnt.com
>
>
>
> 2009/2/21 Scott Tilley <sthed475@telus.net>:
>   
>> Hi Andy
>>
>> Try a 'Linear Feedback Shift Register', Google it...
>>
>> Very simple to implement in a MCU and can be scaled to any size you like.  I
>> wrote some code to do this in PIC assembly years ago that was only a few
>> lines long for a 256bit sequence...
>>
>> Best of all you can predict the outcome and choose a sequence that meets
>> your needs given a unique seed word....
>>
>> Better yet it offers a means of identification over a long term integration
>> :-)
>>
>> 73 Scott
>> VE7TIL
>>
>>
>> Andy Talbot wrote:
>>     
>>> ...but if any mathematicians,  number theorists or cryptographers
>>> should be reading this...
>>>
>>> I've just got a WSPR generating routine running on a PIC / AD9852 /
>>> GPS combination, but I'm looking for a half-decent way of generating a
>>> pseudo random Tx/Rx sequence with defined duty cycle that does not
>>> give the long(ish) consecutive runs of 0's or 1's that MUST ALWAYS
>>> occasionally be seen in a truly random sequence.
>>>
>>> In other words, the sequence needs to be bandpass filtered.
>>>
>>> Before I cop-out and just use a pre-generated set of lookup tables,
>>> take a look at   http://www.g4jnt.com/WSPR_PN_Sequence.pdf
>>> for a fuller description of the mind-bender
>>>
>>>
>>> Andy  G4JNT
>>> www.g4jnt.com
>>>
>>>
>>>
>>>       
>>     
>
>
>